For some time there has been a requirement to be able to locate the position of a radar emitter, especially in a hostile environment, and to be able to passively do so by overflying a theater and detecting emissions from the radar.
Presently, two conventional principles are used for locating the emitter, both relying on the emitter being a stationary point target. Direction-finding techniques exploit the spatial coherence of the electromagnetic radiation, where the airborne (or space-borne) platform possesses two or more spatially separated apertures, whose phase difference between their outputs determine the direction of propagation of the incident electromagnetic field, thus determining from whence it came. The change in direction of arrival over time as the surveillance receiving platform moves determines the location of the emitter by triangulation.
The second conventional method for geolocating a radar exploits Doppler shift, which assumes that the emitter is frequency stable. Assuming that the electromagnetic radiation from an emitter is detected by at least one aperture in the form of an antenna on a flying platform that overflies the emitter, motion of one's own platform or shift creates a Doppler shift versus time along the flight path. It has been found that one can determine the location of the emitter or at least its range based on the Doppler shift.
Such Doppler shift techniques are explained in an article entitled “Analysis of Single Platform Passive Emitter Location With Terrain Data,” by Mark L. Fowler in IEEE Transactions On Aerospace and Electronic Systems, Vol. 37, No. 2, April 2001, and in U.S. Pat. No. 5,870,056 by Mark L. Fowler. Another Doppler triangulation transmitter location system is described in U.S. Pat. No. 5,874,918 by Steven V. Czarnecki, James A. Johnson, Clifford M. Gray, George VerWys and Carl Gerst.
It will be appreciated that all of the above Doppler-frequency techniques require that one know the frequency of the emitter so that one can perform the Doppler frequency calculations.
Note in the Czarnecki et al. patent, the frequency is measured by cross-correlating coherent pulses of the received frequency signal.
These systems are frequently referred to as frequency-difference-of-arrival systems or FDOA systems. In essence, what these systems do is perform a Fast Fourier Transform to be able to identify the frequency of the emitter.
The problem with current passive geolocation systems is that heretofore they have not been able to meet military standards of providing three-meter accuracy, at a 50-nmi range, with processing times of less than three seconds. Even with data taken over long intervals which provides a relatively long baseline, it is not presently possible with the techniques noted above to in any way meet the above requirement.
As will be discussed hereinafter, these emitters, be they pulse-Doppler radars or moving-target-indicator (MTI) radars, have not had sufficiently phase-coherent emissions to permit coherent processing of electromagnetic radiation from these radars.
Moreover, when one has tried to geolocate on such radars using the conventional techniques, the presence of noise and the limitations of aperture size affect the accuracy with which one can measure direction of arrival. Moreover, limitations in how accurately one can measure Doppler shift in the presence of noise prevents the required accuracy.
In general, the accuracy in terms of range for any of the prior geolocation systems is on the order of 10% of the actual range. Thus, for an emitter that is 50 kilometers away from an overflying aircraft, the best range measurements were on the order of 5 kilometers. In general this does not yield sufficiently accurate estimations of the position of the emitter to be able to countermeasure the emitter.
Thus, passively determining the range to an emitter from a single airborne or low earth orbit satellite surveillance platform has been problematic.
By way of further background, the more modern coherent radars tend to use a single ovenized crystal-controlled oscillator as a local time-frequency standard in order to achieve better performance in sub-clutter visibility. The term ovenized refers to a crystal-controlled oscillator whose quartz crystal is placed inside a temperature-controlled oven in order to enhance frequency stability and phase coherence. These oscillators are also used to improve Doppler resolution and in some cases are used in target identification.
Emissions from such radars tend, perhaps unintentionally, to be phase-coherent over periods of time extending to several tens or even hundreds of milliseconds. As will be seen, it is a finding of the subject invention that such phase coherence over hundreds of milliseconds is sufficient to improve geolocation accuracies by two orders of magnitude.
From the radar designers' perspective, modern radars are concerned with local oscillator phase coherence only over the round trip time of the transmitted radar pulse and the return of its echo, a duration on the order of several tens of microseconds to perhaps one millisecond. For suppression of strong, near-in-ground or sea clutter, good phase coherence over a few tens of microseconds has been found to be sufficient.
To radar designers, phase coherence and frequency stability are both separate and necessary specifications of local oscillator performance, with one spanning a time interval that is much different from the other. In their zeal for better radar performance by improving frequency stability and short-term phase coherence, radar designers have inadvertently created coherent radars possessing long-term phase coherence, even though they do not require it.
For example, any corruption of phase that occurs between the time that an echo is received from the last transmitted pulse and the time of transmission of the next pulse will not affect radar performance.